Add Jaguar CD support feasibility study and technical research

Spike document for issue #41 covering Butch ASIC register map,
CD BIOS requirements, disc image formats, existing codebase gaps
(cdintf.c stubbed, BUTCHExec disabled), reference implementations,
the 13-title game library, and a phased implementation roadmap.

Key finding: ~60-70% of CD subsystem infrastructure already exists.
The critical blocker is the stubbed CDIntf disc image reading layer.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: JoeMatt